The present invention relates to waste water treatment, and more particularly to a process for removing phosphorous from waste water involving biological processes and a membrane filter.
BOD, nitrates and phosphates released into the environment cause eutrophication and algae blooming resulting in serious pollution and health problems. Waste water treatment processes attempt to remove BOD, nitrates and phosphates to produce an acceptable effluent.
Conventional processes for removing phosphates from waste water include chemical precipitation and biological methods. In chemical precipitation methods, soluble salts, such as ferrous/ferric chloride or aluminum sulphate, are added to the waste water to form insoluble phosphate metal salts. The waste water, however, contains many different ions which create undesirable side reactions with the precipitants. As a result, and particularly where very low effluent total phosphorus levels are required, these processes may require the addition of 5-6 times the stoichiometric amount of chemicals required to remove the phosphates. Accordingly, these processes result in high chemical costs, high sludge production, and a high level of metallic impurities in the sludge.
In contrast, biological methods use microorganisms to digest the phosphates. For example, U.S. Pat. No. 4,867,883 discusses a process which attempts to create conditions which encourage the selection and growth of Bio-P organisms, a strain of bacteria which have the ability to uptake phosphorus in excess of the amount normally needed for cell growth. The amount of phosphorus removal that can be achieved is directly proportional to the amount of Bio-P organisms in the system. Generally, the process consists of an anaerobic zone, an anoxic zone, an aerobic zone, a clarifier, and a variety of recycles to interconnect the various zones. In a preferred embodiment of the process, there is a denitrified recycle from the anoxic zone to the anaerobic zone, a nitrified recycle from the aerobic zone to the anoxic zone, and an activated sludge recycle from the clarifier to the anoxic zone. In the anaerobic zone, there is BOD assimilation and phosphorus release. Subsequently, in the anoxic and aerobic zones, there is phosphorus uptake. In the clarifier, sludge containing phosphates settles out of the effluent. In some cases, sand filters are employed to try to further reduce the amount of phosphates in the effluent.
One problem with the U.S. Pat. No. ""883 process is that there can be a build-up of phosphates in the system. At the end of the process, a portion of the recycled activated sludge is wasted and is subsequently treated, typically by aerobic or anaerobic digestion processes. This results in a release of phosphorus taken up in the process. This phosphorus is then returned back to the process in the form of digester supernatant. Consequently, this reduces the efficiency of phosphorus removal in the process and results in higher levels of phosphorus in the effluent. A partial solution to this problem is to employ a side stream process called xe2x80x98Phos-Pho Stripxe2x80x99 as described in U.S. Pat. No. 3,654,147. In this process, the activated sludge, which has a high concentration of phosphorus, passes from the clarifier to a phosphorus stripper. In the stripper, phosphorus is released into the filtrate stream by either: creating anaerobic conditions; adjusting the pH; or extended aeration. The resulting phosphate-rich filtrate stream passes to a chemical precipitator. The phosphate-free effluent stream is added to the main effluent stream, the waste stream from the precipitator containing the phosphates is discarded, and the phosphate-depleted activated sludge is returned to the main process.
Another disadvantage with the process in U.S. Pat. No. ""883 is that significant design limitations are imposed by the settling characteristics of the sludge in the clarifier. For example, the process cannot operate at very high process solids levels or high sludge retention times. As a result, the system is generally considered to be inefficient and there is a high generation rate of waste sludge.
A second type of biological treatment is referred to as a membrane bioreactor which can be combined with chemical precipitation techniques. In a simple example, precipitating chemicals are added to an aerobic tank containing or connected to a membrane filter. As above, however, dosages of precipitating chemicals substantially in excess of the stoichiometric amount of phosphates are required to achieve low levels of phosphates in the effluent. This results in excessive sludge generation and the presence of metallic precipitates which increase the rate of membrane fouling or force the operator to operate the system at an inefficient low sludge retention time.
Also relevant to the present invention is U.S. Pat. No. 5,658,458 which discloses a treatment for activated sludge involving the separation of trash and inerts. Generally, the process consists of a screen which removes relatively large pieces of xe2x80x98trashxe2x80x99 and a hydrocyclone which uses a centrifugal force to separate the organics from the inerts. The activated sludge is recycled back to the system and the trash and inerts are discarded.
It is an object of the present invention to remove phosphorous from waste water. In some aspects, the invention provides a process for treating water to remove phosphorous and nitrogen. The process includes three zones, an anaerobic zone, an anoxic zone and an aerobic zone. In the anaerobic zone, an anaerobic mixed liquor has organisms which release phosphorous into the anaerobic mixed liquor and store volatile fatty acids from the anaerobic mixed liquor. In the anoxic zone, an anoxic mixed liquor has organisms which metabolize stored volatile fatty acids, uptake phosphorous and denitrify the anoxic mixed liquor. In the aerobic zone, an aerobic mixed liquor has organisms which metabolize stored volatile fatty acids, uptake phosphorous and nitrify the aerobic mixed liquor.
Water to be treated flows first into the anaerobic zone to join the anaerobic mixed liquor. Anaerobic mixed liquor flows to the anoxic zone to join the anoxic mixed liquor. Anoxic mixed liquor flows both back to the anaerobic zone to join the anaerobic mixed liquor and to the aerobic zone to join the aerobic mixed liquor. The aerobic mixed liquor flows to the anoxic zone to join the anoxic mixed liquor and also contacts the feed side of a membrane filter. The membrane filter treats the aerobic mixed liquor to produce a treated effluent lean in phosphorous, nitrogen, BOD, suspended solids and organisms at a permeate side of the membrane filter and a liquid rich in rejected solids and organisms.
Some or all of the material rejected by the membrane filter is removed from the process. This may be done by locating the membrane filter outside of the aerobic zone and directly removing the liquid rich in rejected solids and organisms from the retentate or feed side of the membrane filter. Alternatively, the membrane filter may be located in the aerobic zone so that the material rejected by the membrane filter mixes with the aerobic mixed liquor. The material rejected by the membrane filter is then removed by removing aerobic mixed liquor. Further alternatively, the liquid rich in material rejected by the membrane filter may be recycled to the anoxic or aerobic zones. The material rejected by the membrane filter is then removed by removing aerobic mixed liquor. Combinations of the first and third methods described above may also be used.
The steps described above are performed substantially continuously and substantially simultaneously. In the anaerobic zone, fermentive bacteria convert BOD into volatile fatty acids. Bio-P organisms use the volatile fatty acids as a carbon source. In doing so, they release phosphorus into the liquor, and store volatile fatty acids for later use. The stored carbon compounds may come from volatile fatty acids produced in the anaerobic zone or from materials produced external to the process or both. For example, upstream waste water fermentation can occur either in prefermentation units specifically designed for this purpose, or inadvertently in the sewage system. Subsequently, in the anoxic and aerobic zones, the Bio-P organisms metabolize the stored volatile fatty acids and uptake phosphates from the liquor. The recycle between the anoxic and anaerobic zones allows the process to operate substantially continuously.
The stream exiting the aerobic zone passes through the membrane filter. In the membrane filter, phosphorus-rich activated sludge, finely suspended colloidal phosphorus, bacteria, and other cellular debris are rejected by the membrane. A waste activated sludge containing material rejected by the membrane filter, optionally combined with aerobic mixed liquor, flows to a sludge management or processing system. A phosphorous lean effluent is produced at the permeate side of the membrane filter. The effluent is also reduced in nitrogen as a result of the anoxic and aerobic zones and the recycle between them.
The membrane filter removes colloidal phosphorus and bacteria which would normally pass through a clarifier. Although the absolute amount of colloidal solids is relatively small, the percentage of phosphorus in the colloids is surprisingly high and its removal results in unexpected low levels of phosphorus in the effluent. With membrane filters to remove biomass from the effluent stream, a fine biomass can be maintained in the anaerobic reactor. This may result in enhanced reaction rates and higher than anticipated release of phosphorus in the anaerobic reactor, with resulting higher uptake of phosphorus in the anoxic and aerobic zones. Further, since the process is not limited by the settling characteristics of the sludge, the process is able to operate at very high process solid levels, preferably with an MLSS between 3 and 30 mg/L and short net hydraulic retention times, preferably between 2 and 12 hours.
The short HRT allows increased throughput of waste water for a given reactor size. In addition, since the design avoids chemical precipitation of phosphates upstream of the membrane filters, there is reduced membrane fouling which further enhances the performance of the process. Moreover, contaminants in the sludge resulting from precipitating chemicals are reduced permitting the system to operate at a high sludge age. At high sludge retention times, preferably between 10 and 30 days, an unexpected significant crystalline phosphorus accumulation occurs in the biomass, effectively removing phosphorus from the system. As well, there is lower net sludge generation.
The processes described above optionally includes one of two side stream processes. In a first side stream process, a liquid lean in solids but containing phosphorous is extracted from the anaerobic mixed liquor. Phosphorous is precipitated from that liquid to produce a phosphorous lean liquid which leaves the process as effluent or is returned to the anoxic or aerobic zones. In a second side stream process, anaerobic mixed liquor is removed to a reaction zone and treated to form a liquid rich in insoluble phosphates. The liquid rich in insoluble phosphates is treated in a hydrocyclone to separate out insoluble phosphates and create a liquid lean in insoluble phosphates. The liquid lean in insoluble phosphates is returned to the anoxic zone.